Bridges, elevated highways, and other large structures supported on load bearing columns are often constructed in areas where earthquake protection for the structure is required. The structural integrity of these structures is highly dependent on the capacity of the load bearing columns to survive the stresses imposed during an earthquake. A structure may be able to withstand the loss of one or more load bearing columns, however, each failure increases the load on the rest of the structure, and makes it more likely that the entire structure will fail. Thus, it is critical to prevent a load bearing column from failing under the forces and moments generated within the column during an earthquake. These loads include horizontal, and vertical forces as well as twisting and bending moments.
The development of earthquake protection for buildings has heretofore focused primarily on methods to isolate the structure from the foundation. Base isolation is the name given to these methods. A building supported by a base isolation system will "float" on its foundation. Additionally, damping systems are also employed to reduce any motion the structure may develop. See generally U.S. Pat. No. 3,606,704 (Denton); U.S. Pat. No. 3,794,227(Smedley et. al.); U.S. Pat. No. 4,860,507 (Garza-Tamaz); U.S. Pat. No. 5,386,671 (Hu et. al). Base isolation has proven to be an effective method of protecting buildings from earthquake loads. Buildings using base isolation are supported by a foundation with a relatively large area (foot print) with the typical building having a square or rectangular shape and four external load bearing walls. Thus, the earthquake forces are spread over a large area. Additionally, a building, even one on a base isolation system, will be stable under most loads. A building will only become unstable when the building's center of gravity (approximately the building's geometric center) is moved so that the center of gravity lies outside the vertical plane of one of the exterior load bearing walls. If a building becomes unstable, then the building will tip over; however, the typical building would be unlikely to be able to survive the loading which would generate the forces necessary to move a building's center of gravity the distance necessary to cause the building to topple.
Despite the progress in developing earthquake dampers for buildings, there have not been any earthquake dampers developed for bridges, elevated highways, or similar large structures which has proven effective for use in the load bearing column itself. Additionally, typical construction methods use either a single column or a single row of columns to support a cross-beam or pier head. This cross-beam or pier head supports the rest of the structure. Some earthquake protection systems have been developed which act as a form of base isolation. These devices have been placed between the cross-beam or pier head and the girder structure of the bridge. See U.S. Pat. No. 3,986,222 (Miyazaki et. al.) And U.S. Pat. No. 4,720,882 (Gallo). Earthquake protection systems located between the beam or pier and the girder structure may provide some protection for the girder structure, however, the load bearing column and the cross-beam or pier head, critical structural members located between the foundation and the shock dampers, are left unprotected.
Designing and constructing earthquake protection for these columns is more difficult than designing and constructing protection for a building. This difficulty arises because of the following differences between a column and a structure: 1) the earthquake loads in a building are spread over a large number of load bearing members compared to a small number for a bridge, 2) the earthquake loads in a building are spread over a relativity large area compared to the small cross-section of a column, 3) a structure has a large range of stability compared to a column, and 4) a structure can "float" on a base isolation system installed between the building and its foundation, typical columns must be fixed to their foundation for proper support.
Earthquake protection for load bearing columns currently consists of designing the column to withstand all the forces and moments generated during an earthquake. Designing the column to withstand earthquake forces and moments has several drawbacks. The principal problems with this approach are a) added cost of building the stronger pillar, and b) added cost of designing and building the full structure to withstand earthquake loads or cost of placing earthquake dampers or isolators between the cross-beam or pier head and the rest of the structure. Also, the earthquake dampers/isolators which have been developed for use between the cross-beam or pier head and the structure provide earthquake load damping primarily only in a single direction, whereas earthquake forces ordinarily develop in multiple directions, e.g., both horizontal and vertical directions. See U.S. Pat. No. 4,720,882(Gallo) and U.S. Pat. No. 3,986,222 (Miyazaki et al).
Unfortunately, recent earthquakes have demonstrated the deficiencies of existing methods of "earthquake proofing" large structures, and consequently have shown the the need to protect load bearing columns from failure during earthquakes. Thus, there is a need for an earthquake damper/isolator which can be used in both new columns and retrofitted into existing columns to protect both the column and supported structure from earthquake forces and moments regardless of direction.